THE BRITTLE-DUCTILE TRANSITION IN HIGH-LEVEL GRANITIC MAGMAS - MATERIAL CONSTRAINTS

Granitic magmas residing at high levels in the continental crust may b e subjected to stress transients on a wide range of time-scales. The o rigins of the stress transients may be internal, deriving from the vol ume evolution of the magmatic system, or external such as the rapid re lease of stresses during tectonic and volcanically derived earthquakes . The strain response of the granitic magma is critically dependent up on its viscosity and the magnitude and time-scale of the application o r release of the stress. The response of the magma may be either ducti le or brittle. Prediction of the expected response of the magma requir es an accurate and reliable method for determining the location in tem perature and time-scale of the transition of the response of the melt from ductile to brittle bu analogy to the glass transition determined by frequently domain relaxation studies, The application of the Maxwel l criterion, which treats the magma as a viscoelastic medium in the li near stress-strain regime, provides such a method Application of this method requires accurate viscosity and elastic modulus data for the ma g-ma. The consequence of intersecting the glass transition in a system where the total strains are significant is brittle failure of the mel t phase. Here, the material constraints on the brittle-ductile transit ion of granitic magma are reviewed.